Describe what happens when a muscle contracts.

When a muscle contracts, it is performing a complex biomechanical process that involves the interaction of muscle fibers, nerves, and energy sources. Here's what happens step by step:

1. **Neuromuscular Junction Activation**: The process begins when the nervous system sends a signal (a nerve impulse) to the muscles, telling them to contract. This occurs at the neuromuscular junction, where a nerve cell connects with a muscle fiber.

2. **Release of Acetylcholine**: The nerve impulse reaches the end of the nerve cell (axon terminal), causing the release of a neurotransmitter called acetylcholine (ACh) into the synaptic cleft (the small gap between the nerve and muscle).

3. **Excitation of the Muscle Cell**: The acetylcholine binds to receptors on the muscle cell's membrane (sarcolemma), which opens ion channels and allows sodium ions to rush into the muscle cell. This causes a change in electrical charge across the membrane (depolarization), which triggers an action potential that travels along the sarcolemma and into the muscle fiber's interior through structures called transverse (T) tubules.

4. **Calcium Release**: The action potential continues into the sarcoplasmic reticulum (SR), a specialized form of the endoplasmic reticulum in muscle cells. In response, the SR releases calcium ions into the sarcoplasm (the cytoplasm of muscle cells).

5. **Actin and Myosin Interaction**: The calcium binds to a protein called troponin, which causes a positional change in another protein, tropomyosin, that was covering binding sites on the actin filaments within the muscle cells. With these sites now exposed, the myosin heads, which are part of the myosin thick filaments, can bind to actin to form a cross-bridge.

6. **Power Stroke**: Using energy from ATP (adenosine triphosphate), the myosin heads pivot, pulling the actin filaments toward the center of the sarcomere, the functional unit of a muscle fiber. This action shortens the muscle fiber, causing muscle contraction.

7. **Detachment and Re-cocking**: Another ATP molecule binds to the myosin head, causing it to detach from the actin. The ATP is then hydrolyzed (broken down), and the energy released re-cocks the myosin head, priming it for another cycle of cross-bridge formation and power stroke if the stimulation continues.

8. **Relaxation**: When the nerve impulse stops, acetylcholine is broken down by an enzyme called acetylcholinesterase, calcium ions are pumped back into the SR, and the muscle fiber relaxes and lengthens back to its resting state.